1. Field of the Invention
The present invention relates to a three-wave common antenna for use in vehicles which uses a double sleeve system so as to transmit and/or receive three types of radio waves, that is, AM waves, FM waves, and telephone waves, via a single antenna element.
2. Prior Art
The AM and FM broadcast waves and telephone waves which are transmitted and received by wireless telephones used in automobiles have greatly different frequencies. At present, therefore, an antenna for receiving AM and FM waves and an antenna for transmitting and receiving telephone waves are installed separately at different locations on a vehicle body.
FIGS. 5 (a) and 5 (b) respectively show a conventional example of an antenna for AM/FM reception and a conventional example of an antenna for telephone transmission and reception.
The AM/FM receiving antenna shown in FIG. 5 (a) has an overall length of 790 to 840 mm and uses a tapered rod 1 which has a slender tip end and a thick base end. This antenna is set at a length of roughly .lambda./4 so that it can act as a monopole antenna with respect to FM waves. The antenna is also set at a more or less standard length so that the required sensitivity can be obtained with respect to AM waves. If the overall length of the antenna is greater than 790 to 840 mm, the sensitivity with respect to AM waves is improved, but the sensitivity with respect to FM waves drops, especially on the high-frequency side of the FM band. If the overall length of the antenna is less than 790 to 840 mm, the sensitivity with respect to AM waves drops abruptly, and the sensitivity with respect to FM waves also drops, especially on the low-frequency side.
The telephone transmitting and receiving antenna shown in FIG. 5 (b) uses a so-called two-stage collinear system. In other words, a phase coil 3 is installed roughly at the middle of the antenna rod 2. The overall length of this antenna is 250 to 300 mm. This telephone antenna has a sensitivity of +3 dB at the standard dipole antenna ratio.
The AM/FM receiving antenna shown in FIG. 5 (a) is usually mounted to one of the fenders of a vehicle body, and the telephone antenna shown in FIG. 5 (b) is ordinarily attached to the trunk of the vehicle or to the edge of the roof.
However, the separate installation of the two different types of antennas as described above has a deleterious effect on the design of the vehicle. The additional attachment work required is a nuisance, too.
FIG. 5 (c) shows one example of a three-wave common antenna which includes an AM/FM receiving antenna combined with a telephone transmitting and receiving antenna. Thus, this antenna is in a single antenna unit to solve the above-described problems. In this antenna, the antenna section is formed as a so-called three-stage collinear system with respect to the telephone waves, and the overall length of the antenna rod 4 is 580 to 630 mm. Two phase coils 5 and 6 are installed at intermediate points of the antenna rod 4, and a wave splitter 7 which is used to separate AM/FM waves from telephone waves is installed on the base of the antenna. Connectors 8 and 9 are installed on branched cable tips and respectively connected to an AM/FM radio receiver and wireless telephone set.